Method of filtering submicron particles with gel lattice membrane filter

ABSTRACT

A method of making a solid filter material which filters a predetermined wavelength band from a broader spectrum of radiation is provided. The method includes creating a colloidal structure composed of particles dispersed within a medium, and introducing a solvent thereto. Thereafter, the solvent is evaporated and the remaining structure solidifies into a solid crystalline array. The particles can also be fused together by polymerization using one of several methods which are provided. In another embodiment, methods of filtering submicron particles have been developed which consists of establishing a gel membrane from a crystalline colloidal array with an interstice size less than or equal to the particles to be filtered are disclosed. The gel membrane may employ anisotropic interstices of submicron size and is stretchable or compressible mechanically. The method also includes stacking a plurality of gel membrane filters so that the material to be filtered sequentially flows through the interstices of the filters leaving different size submicron particles on different levels of said filters. Another embodiment of the invention has a plurality of particles having a positive or negative charge in a lattice and has oppositely charged mobile particles movable into and out of the interstices of the gel membrane. An electric field is employed to move the mobile particles to close or open the interstices of the lattice. The mobile particles can also pump material through the interstices. Decorative uses of the materials are also disclosed.

This application is a continuation of U.S. application Ser. No.08/151,476, filed Nov. 12, 1993, entitled "METHOD OF FILTERING SUBMICRONPARTICLES AND ASSOCIATED PRODUCT", now abandoned, which is acontinuation-in-part of U.S. application Ser. No. 07/571,251, filed Aug.22, 1990, entitled "METHOD OF MAKING SOLID CRYSTALLINE NARROW BANDRADIATION FILTER AND RELATED DEVICE, now U.S. Pat. No. 5,281,370.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a method of creating filterswhich may be used to select and/or reject predetermined frequencies ofelectromagnetic radiation. The invention relates more particularly to amethod of making solid crystalline materials in which colloidalelectrically charged particles form an ordered dispersion and aresolidified into a hardened material which has certain predeterminedfiltering characteristics.

In another embodiment of this present invention, submicron periodicmaterials are employed for size selective particle and molecularfiltration based in part on the self-assembly properties of crystallinecolloidal arrays. In this embodiment, the material may be passive oractive and have switchable filtration properties.

2. Description of the Prior Art

It has been recognized that colloidal dispersions of polymer particlesin various solvents can form crystalline structures having latticespacings comparable to the wavelength of ultraviolet, visible andinfrared radiation. Bragg diffraction techniques have been used toexamine these polymer colloidal crystals with a view towards identifyingtheir interparticle spacing, lattice parameters and phase transitions.U.S. Pat. No. 4,627,689 discloses a crystalline narrow band radiationfilter which is made by forming a highly ordered crystalline colloidalstructure within a cell. The crystalline colloidal structure is formedby dispersing electrically charged particles, for example, polystyreneparticles within an appropriate solvent. U.S. Pat. No. 4,632,517 alsodiscloses a narrow wavelength band filtering device created by forming ahighly ordered crystalline colloidal structure within a cell.

More recently, it has been known that these crystalline structures canbe very useful and that such structures have many practical applicationsfor filter devices. In many instances it is necessary or desirable tofilter out narrow bands of selected wavelengths from a broader spectrumof incident radiation while permitting the transmission of adjacentwavelengths. High spectral-purity commercial monochromators which areavailable for this purpose generally use a plurality of gratings andprisms. However, such devices are extremely complex, bulky andexpensive. U.S. Pat. No. 4,632,517 discloses another crystallinecolloidal narrow band radiation filter which may involve polystyreneparticles. The device of this patent forms the basis for a mechanicallysimple and highly efficient monochromator. It has application inimproved systems for investigating Raman or emission spectra of selectedsample materials. U.S. Pat. No. 4,632,517 disclosed a type of solidstructure in that with a lattice spacing gradient being formed and aspart of this process a "freezing" of certain conditions is achievedusing polymerization techniques. However, this suggestion did notdisclose the unique aspects of the method and product of the presentinvention for forming filtering devices which are entirely solid andself-supporting.

The disclosures of U.S. Pat. Nos. 4,627,689 and 4,632,517, are expresslyincorporated herein by reference.

Other filtering devices have also been known. See, for example, U.S.Pat. No. 4,803,688 which discloses ordered colloidal suspension opticaldevices. This patent relates to the addition of a water based polymer toa colloidal structure.

A nonlinear optical device which has a high speed switching capabilityat high radiation intensities and which can be used for rejectingcertain undesired wavelength bands from such high intensity radiation isdisclosed in Asher U.S. patent application Ser. No. 07/999,487 filedDec. 30, 1992, entitled "A Method of Making An Optically NonlinearSwitched Optical Device and Related Devices", now U.S. Pat. No.5,452,123. This application is owned by the assignee of the presentapplication. This application discloses a method for making a nonlinearoptical device and a related optical device. The filter effectivelyresists transmission of about 99.9% of radiation from a wavelength band.The material may operate as a high speed optical switch in that itbecomes opaque to radiation within several nanoseconds.

Although an allusion was made to solid devices in some of theabove-mentioned prior art, these patents involve crystalline colloidalstructures which are not solids and are not self-supporting. Because oftheir high peak absorbance value, state-of-the-art colloidal crystallinearray filters may be widely used for eye protection and sensorprotection. However, a more rugged filter would obviously have widerapplication. As a result, there has been a need for a solid filter.Solids provide better mechanical stability and machinability.Non-solids, on the other hand, are subject to becoming disordered uponvibration and shock. In addition, liquid media can undergo phasetransitions quite easily by freezing or boiling and this may often beundesirable.

For these reasons a solid structure is more desirable in manyapplications. For example, solid filtering devices are often necessaryfor filtering out certain bands of radiation in aviation and spacetravel, as they provide mechanical rigidity and this allows for agreater range of design features. It has heretofore been an extremelydifficult polymer chemistry problem to create such a solid filter. Seegenerally U.S. Pat. No. 5,131,736.

Despite all of these optical radiation filtering devices, there stillremains a need for a porous polymerized film adapted to be employed forthe filtration of solid or liquid materials of relatively small size.Such a filter can be of fixed or adjustable porosity. There also remainsa need for the development of filters from submicron periodic materialsthat are polymerized into porous gel membrane filters and can be usedfor size selective submicron particle and molecular filtration. Therealso remains a need to provide a simple method of creating a gelmembrane filter whose interstices allow separation of predetermined sizesubmicron material which may be a solid, solids in a liquid, liquid outof liquids or a virus in a liquid.

SUMMARY OF THE INVENTION

These and other needs are satisfied by the present invention whichprovides a simple and relatively inexpensive method of creating a solidnarrow band radiation filter and related devices. The narrow bandradiation filter selectively and effectively filters a narrow band ofwavelengths from a broader spectrum of incident radiation whiletransmitting adjacent wavelengths to a high degree. For example, afilter can be produced in accordance with the present invention whichfilters out greater than about 99 to 99.999% of a wavelength band ofabout 50 to 150 Å while transmitting more than about 70 to 90% of theintensity of remaining wavelengths.

A method of making the filter is also disclosed in which a crystallinestructure is created which is composed of particles dispersed in aliquid medium. As used herein, "particles" includes any shape suitablefor the desired filtering need, but preferably the particles for thepresent invention will be spheres. In accordance with one aspect of theinvention, a solvent is introduced into the crystal structure that fusesthe particles together. Thereafter, the solvent is evaporated tocondense the particles into a three-dimensional array having a highlyperiodic lattice spacing. The lattice spacing is created such that itcan diffract a predetermined wavelength band.

As noted hereinbefore, the particles are fused together and a geometricordering occurs. The lattice structure exists largely due to electricalrepulsive forces between the particles which each have a charge of thesame polarity. Several different methods of fusing the particlestogether are disclosed which are set forth in further detailhereinafter.

One aspect of the method of securing the particles in the desiredrelative position together involves polymerization of the mediumsurrounding the particles to fix the particles in the desired relativespaced relationship to each other. A particular method of suchpolymerization includes adding acrylamide or bisacrylamide andpreferably a nonionic UV photoinitiator to a colloidal solutioncontained between two quartz plates. Ultraviolet light is then utilizedto initiate the polymerization.

In accordance with another embodiment of the method a polymer solutionis introduced into the region around the polystyrene spheres. Thispolymer rigidizes the medium and fixes the sphere positions. The polymermay be an organic or inorganic material.

An alternate method includes providing particles, which may bepolystyrene, polymethyl methacrylate or silica spheres, for example, incolloidal form with a coating which provides a film of polymerizingresidue on the surfaces of the particles. The ordered colloidal array isformed and is then solidified by polymerizing together the adjacentsphere surfaces.

Another method of the invention involves packing the particles utilizingan electric field to attract the particles and further enhance theordering of the array. Subsequent to achieving this ordering,polymerizing may be effected with the assistance of the electric fieldwhich will electrochemically initiate polymerization of material, suchas acrylamide and bisacrylamide, for example.

The device resulting from the methods of this invention can form thebasis for a mechanically simple and highly efficient filter which isuseful in many applications, such as, for example, sensor protection,eye protection, scientific instrumentation and medical instrumentationin laser surgery. Such filters can also eliminate the need for dichroicmirrors in optic technology. Overall, the device can be used with anyproduct in which the disclosed radiation filtering characteristics aredesirable.

In another aspect of the invention, a method and product of the presentinvention involves filtering submicron particles has been developedwhich consists of establishing a gel membrane filter, from a crystallinecolloidal array, with an interstitial size less than or equal to theparticles to be filtered. The material to be filtered impinges upon thegel membrane filter and the membrane filter resists passage of thematerial through the membrane interstices. In one embodiment, theinterstices are of a fixed size. In another embodiment, the membranefilter openings are of an adjustable size. The gel membrane filter mayemploy anisotropic interstices of submicron size. The gel membranefilter can be mechanically stretched by applying force to the gelmembrane filter in one or two directions. This can be done bymechanically clamping and stretching as well known to those skilled inthe art.

Another embodiment of the invention has a plurality of charged particleshaving a positive or negative charge with mobile oppositely chargedparticles disposed in the interstices of the gel membrane filter. Anelectric field can be employed to move the mobile particles between anopen position which is substantially nonobstructing to said intersticesand a closed position which obstructs a substantially greater portion ofsaid interstices. As used herein, the term "electromagnetic field" isdeemed herein to be an electric field.

As used herein, the term "material" or "materials", when used to referto the present invention being employed as a filter or pump, shall meanthe material being subjected to filtration or pumping action, includingboth the particles and molecules which are permitted to pass through thefilter or restrain from passage through the filter. The invention alsoincludes stacking a plurality of gel membrane filters so that a portionof the material to be filtered sequentially flows through theinterstices of the filters and leaves different size submicron materialon different levels of the filters.

It is an object of the invention to provide a method of creating a solidfilter which can effectively filter a predetermined narrow wavelengthband from a broader spectrum of incident radiation.

It is an object of the present invention to provide an inexpensive,simple method of creating a solid crystalline structure havingpredetermined submicron filtering characteristics.

It is another object of this invention to provide a simple method ofcreating a gel membrane filter whose interstices allow separation ofpredetermined size submicron material which may be a solid, solids in aliquid, liquid out of liquids or a virus in a liquid.

It is yet another object of this invention to provide a simple gelmembrane filter whose interstices are stretchable mechanically to filtersubmicron material of different submicron sizes.

It is yet another object of this invention to provide a simple gelmembrane filter with fixed size interstices.

It is yet another object of this invention to provide a simple gelmembrane filter which is stretchable and usable to filter anisotropicparticles.

It is yet another object of this invention to provide a simple gelmembrane filter with both fixed and mobile particles.

It is yet another object of this invention to provide a simple gelmembrane filter that is hardened by backfilling and useful in otherproducts.

These and other objects of the invention will be more fully understoodfrom the following description of the invention, with reference to theillustrations appended hereto.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of colloidal particles dispersedwithin a medium prior to ordering of the particles in accordance withone aspect of the present invention.

FIG. 2 is a schematic illustration of a solid crystalline array ofpolystyrene particles created in accordance with the method of thepresent invention.

FIG. 3 is a schematic illustration of the electrophoretic method oforganizing and polymerizing the particles in a packed array.

FIG. 3A is a cross section of the array taken along line IIIA--IIIA ofFIG. 3.

FIG. 4 is a schematic illustration of the angle at which radiation isdiffracted using the filter of the present invention.

FIG. 5 is a representation of a spectrum indicating the narrow bandwavelengths which can be filtered using devices created in accordancewith the present invention.

FIG. 6 shows schematically a crystalline colloidal array polymerizedinto a porous gel membrane filter.

FIG. 7 is a schematic cross-section in the system fabricated as a stackof membranes that serve as a size selective separation system.

FIG. 8 is a schematic cross-section wherein stretching or compressingthe gel membrane filter causes the lattice constant to asymmetricallychange so that different size submicron material of different sizes andshapes can be filtered.

FIG. 9 is a schematic illustration of an embodiment of the gel membranefilter which has a fixed lattice of negatively charged particles and amobile array of positively charged particles and mobile particles in theclosed position.

FIG. 10 shows the embodiment of FIG. 9 under the influence of anelectric field E with the mobile particles in an open position.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to FIG. 1, there is shown a schematic illustration of agroup of particles 12 prior to ordering of the array which is discussedhereinafter. Particles 12 are interdisposed within a medium 14. As willbe discussed more fully hereinafter, the filtering characteristics ofthe filtering device so created may be varied by controlling the spacingbetween the particles 12 or by altering the shape and size of theparticles 12.

Although not limiting the invention, in a preferred form, particles 12are preferably composed of polystyrene, polymethylmethacrylate, silicondioxide, aluminum oxide, fluorinated polymers, for example Teflon, orother suitable materials which are generally uniform in size andelectrical charge. The material chosen depends upon the optimum degreeof ordering desired in the resulting lattice. The higher the ordering ofthe lattice structure, the narrower the wavelength band which may befiltered with the resulting filtering device. Other parameters alsoaffect filtering characteristics such as particle size and shape.Particles 12 used in the method of this invention preferably have adiameter between about 10 nanometers to 10 microns. These particles 12may be synthesized using the technique disclosed hereinbelow.Alternatively they may be obtained commercially from Polysciences, Inc.of Warrington, Pa.

The particles 12 are generally stored in a dispersion including adetergent and relatively small amounts of an electrolyte. They are firstcleaned of the electrolyte and surfactant impurities. This cleaning maybe accomplished by dialysis against a bath of doubly distilled watercontaining a mixed bed ion exchange resin. After dialysis, the particles12 may be stored in a bath of, preferably, 10% by weight suspension ofion exchange resin called Analytical Grade AG501X8 mixed bed resinobtainable from Bio-Rad of Richmond, Calif. The ion exchange resinshould be cleaned prior to use through a suitable procedure such as thattaught by Vanderhoff, et al. in the Journal of Colloid InterfaceScience, 1968, 28,336-337.

In the form illustrated in FIG. 1, the particles 12 are contained in amedium 14 which may be water, glycerol, ethylene glycol, methanol,ethanol, dimethyl sulfoxide, phenyl methyl sulfoxide, dioxane,dimethylformamide, polyethylene glycol, or glycerine, or any materialpossessing similar properties. The particles 12 within medium 14 in theform shown are placed in a generally rectangular chamber 16 which issealed by cover 28. Chamber 16 is, preferably, composed of quartz, LEXANor LEXAN-coated glass. Chamber 16 has bottom portion 18, and upstandingsidewalls 20, 22, 24 and 26. The suspension composed of particles 12 andmedium 14 is diluted with deionized, doubly distilled water to provide apartial volume fraction in the range of about 0.5 to 75 percent. Chamber16 is then sealed with air-tight cover 28. Sealed chamber 16 is thenplaced in room temperature water 30 in outer container 32 for a periodof time adequate to allow the array to crystallize. This environmentshould also be perturbation-free. Geometric ordering of the crystallinestructure then occurs.

FIG. 2 shows chamber 16 after removal from outer container 32. Theparticles 12 are packed in an ordered array 11 within chamber 16.

Turning now to further details of the method of the present invention,various aspects of the method of forming a solid crystalline structureare discussed.

In accordance with one aspect of the invention, any suitable solventsuch as benzene, toluene and chloroform is added to preferably a polymerlatex solution and this is added to medium 14 (FIG. 1) to fuse theparticles 12 together and create an ordered array 11 as shown in FIG. 2.Preferably, polystyrene or polymethyl methacrylate spheres of about 10nanometers to 10 microns in diameter may be used. However, any suitableparticle with a polymer outer shell may be used within the method of thepresent invention. The solvent, when added, serves to solubilize andswell the outer shells of particles 12.

Thereafter, the solvent medium 14 is removed. A suitable method ofremoval of the solvent medium 14 is preferably by a gentle evaporationwherein chamber 16 is at a temperature between about 20° and 30° C.until the desired evaporation takes place.

During solvent evaporation, the particles 12 condense into an orderedthree-dimensional array 11 and the surfaces of the particles 12 arefused to one another. The resulting solid array 11 can then be removedfrom chamber 16. This array 11 constitutes a film which isself-supporting. The film diffracts and filters radiation of specificpredetermined wavelengths. The wavelengths at which such a film iseffective depend upon the resulting lattice structure, however, thewavelength bands may be on the order of about 2000 to 15,000 Å. In otherwords, the film can be effective from the ultraviolet region through thevisible spectral region and then into and through the infrared region.

EXAMPLE 1

A crystalline colloidal structure was created by a method discussedhereinbelow. This method may be used to create a filtering device whichBragg diffracts a relatively narrow wavelength band with a highrejection ratio. Spheres 12 having a diameter of between about 200 and250 nanometers and a density of between about 1 and 1.1 were used. Thespheres 12 were added to a medium 14 of water containing about 0.1percent toluene. Spheres 12 were added to a total volume density ofabout 10¹³ to 10¹⁴ particles per cubic centimeter.

This suspension was sealed in a chamber 16 (FIG. 1) made of quartz,having internal chamber dimensions of about 5 cm×5 cm×0.5 mm. Thesolvent was then evaporated by placing the chamber at room temperaturefor about twenty hours. A solid crystalline structure was formedtherein, as evidenced by a change from a cloudy appearance to aniridescent appearance.

The solid crystalline structure created from array 11 was then removedfrom chamber 16. The structure so formed was determined to Braggdiffract above about 99 to 99.999% of light at a maximum of about 4800to 5200 angstroms wide wavelength band centered at about 5000 angstromswhile passing about 60 to 80% of the light at adjacent wavelengths.

The filtering device so produced as a narrow band filter would be quiteuseful for many applications, such as goggles for filtering laser lightfrom a pilot's eyes or for a windshield on an airplane or space vehicle.Alternatively, the material may be used in rejecting a narrow wavelengthband for scientific instrumentation or in the field of optics.

In accordance with another aspect of the invention a method forpreparing a solid filter by polymerization of the medium around thespheres involves addition of at least one of acrylamide andbisacrylamide and an ultraviolet photoinitiator to a colloidal solutionsuch as that described hereinbefore. The solution is preferablycontained between two quartz plates. The solution is then exposed toultraviolet radiation which effects the desired polymerization.

In accordance with yet another aspect of the invention, a fabricationmethod for production of monodisperse silica spheres to be used toconstruct a filtering device in accordance with the method of thepresent invention will be disclosed. Silica particles having sulfonategroups on their surfaces were used. A preferred sulfonate material is2-(4 chlorosulfonylphenyl)ethyl trimethoxy silane. The resultantsulfonate groups on the surface of particles are strong acid groupswhich dissociate to provide negative charges on the surface of theparticles, which in this case are preferably spherical. It should beunderstood, however, that positively charged particles in lieu ofnegative particles may also be utilized within the scope of the presentinvention.

Furthermore, in accordance with this aspect of the invention, particlesself-assemble into a three-dimensional array which will diffract light.The medium in which particles are dispersed may be adjusted to vary theinterparticle interactions. As the medium evaporates, the particles havea high density within the medium. The adjustments may be made byincluding in the medium compounds such as NaCl, other salts or morebroadly (a) any ionic compound or (b) any nonionic compounds havingdifferent dielectric constants. Other compounds such as styrene orsilicone oil, for example, leave a residue of a material around theparticles. This residue will not evaporate when the remainder of mediumevaporates and it will tend to seal the spaces between the particles.This method results in a three-dimensional array having a high particlevolume fraction. The material solidifies to form a homogenous solidcrystalline structure which diffracts radiation. While we have usedacidic materials on the surface of the particles, it is also within thescope of the present invention to use basic materials for this purposesuch as quaternary amines, for example. One of the primaryconsiderations is that the particles be electrically charged and theresidue material adheres to the surface thereof.

EXAMPLE 2

Silica spheres were produced by polymerizing tetraethyl orthosilicate ina water-ethanol-ammonium hydroxide mixture using the Stober process. Thespheres were then silanized with 2-(4 chlorosulfonylphenyl)ethyltrimethoxy silane. The spheres were allowed to self-assemble into athree-dimensional array 11 (FIG. 2) in the manner hereinbefore discussedwithin a medium such as water. Thereafter, evaporation was thenfacilitated. The array was removed from chamber 16 and a homogenoussolid crystalline structure was present. The structure so created wasdetermined to Bragg diffract about 99 to 99.999% of the light at amaximum of about 4800 to 5200 angstroms wide wavelength band centered atabout 5000 angstroms, while passing about 60 to 80% of the light atadjacent wavelengths.

In accordance with another aspect of the invention, a method isdescribed with reference to FIG. 3. Polystyrene particles 34 areintroduced into chamber 35 which is composed of preferably two SnO₂-coated quartz plates, 41 and 43. Particles 34 are sandwiched betweenplates 41 and 43. The chamber 35 contains a medium selected from thegroup consisting of water, methanol and ethanol, and a monomer such asacrylamide, bisacrylamide, methyl methacrylate or hydroxy methylmethacrylate. Chamber 35 has a suitable electric field placed across itas schematically shown by wire 37 and voltage source 39. The electricfield causes upper plate 41 to be negatively charged and lower plate 43to be positively charged. The potential across the chamber 35 ispreferably about 20 to 30 volts/cm. The field is preferably applied tochamber 35 for about 5 to 10 minutes. The particles 34, being negativelycharged, are attracted in the direction 47 due to the electric field.They migrate towards lower plate 43 and are packed in an ordered arrayagainst lower plate 43 in the manner shown in FIGS. 3 and 3A. Particles34 migrate due to negatively charged sulfonates on their surfaces. Ofcourse, it should be understood that positively charged particles couldbe used which would then require an appropriate adjustment in theorientation of the electric field. The surface of the particles may bepositively charged using quaternary amines on the surface of theparticles in which case they would migrate towards the oppositeelectrode. After about 5 to 10 minutes, the particles 34 become packedin the array designated generally as 49 (FIG. 3). The monomer in theliquid medium polymerizes around the particles. The polymerization maybe initiated either chemically or electrochemically. The structurethereafter is gelled and solidified as the medium evaporates. A morerigid solid can be obtained by allowing the liquid medium to evaporateand condense the ordered particles into a solid array. The resultingfilter is capable of diffracting or filtering radiation in accordancewith lattice spacing of the array 49.

EXAMPLE 3

An electrophoretic formation method as discussed hereinbefore waspracticed employing polystyrene spheres. Spheres having a diameter ofabout 200 to 250 nanometers and a density of between about 1 and 1.1were used. Spheres 34 (FIG. 3) were placed in a medium 36 of watercontaining about 0.1 percent toluene. The density of spheres 34 withinmedium 36 was between about 10¹² and 10¹⁴. The suspension was placed inchamber 35 made of tin oxide coated glass and having interior dimensionsof about 50 millimeters×50 millimeters×0.5 millimeters.

An electric field of about 20 to 30 volts/cm was placed across chamber35. The electric field caused the negatively-charged spheres 34 tomigrate towards positive end 43. This was allowed to occur for betweenabout 5 and 10 minutes. Thereafter, evaporation was facilitated byplacing chamber 35 in an atmosphere of air kept at about roomtemperature, for about 16 to 20 hours. A filter film having a thicknessof about 0.01 to 0.1 mm was produced which was determined to Braggdiffract above about 99 to 99.999% of light at a maximum of about 4800to 5200 angstroms wide wavelength band centered at about 5000 angstromswhile passing about 60 to 80% of the light at adjacent wavelengths.

It should be understood that the electrophoretic method disclosed hereinis not the only method of packing the particles which are spheres in theexemplary embodiment. There are other methods known to those skilled inthe art including gravitational settling and centrifugal settling.

With reference to FIGS. 4 and 5, the performance of the filtering deviceis illustrated. In FIG. 4, a beam 50 of electromagnetic radiation isincident upon a filtering device 52, made in accordance with the presentinvention, at an angle A. A transmitted beam 54 departs the filteringdevice 52 at a substantially equal angle B. A beam 56 of a narrowwavelength band is Bragg diffracted from the filtering device 52 at anangle C. In this manner, the beam 56 is effectively filtered from beam50. The wavelength of beam 56 satisfies the Bragg diffraction equation:

    nλ=2d sin A

wherein "d" represents the lattice spacing between each sphere 12(FIG. 1) within the solid structure, "n" represents any integer whichwill preferably be about 1, "A" is angle A, and represents wavelength.

The curve 60 of FIG. 5 illustrates that the wavelength band which isBragg diffracted by the filtering device 52 made in accordance with thepresent invention may be quite narrow. Referring still to FIG. 5, curve73 depicts that the central wavelength of the narrow wavelength bandfiltered by filtering device 52 is dependent upon the incident angle asdetermined by the Bragg diffraction equation set forth hereinbefore.Accordingly, it will be appreciated that the filtering device is"tunable" to filter a range of different wavelength bands, bycontrolling and appropriately adjusting the orientation between thefiltering device 52 and the incident electromagnetic radiation.

The embodiment of the invention directed toward filtering solid materialwill be considered in further detail with reference to FIGS. 6-10.

FIG. 6 shows schematically a crystalline colloidal array which has beenpolymerized into a porous gel membrane. This gel membrane is based onthe self-assembly properties of crystalline colloidal arrays. Thecrystalline colloidal arrays consist of polymers or inorganic particleswhich are preferably generally spherical and contain ionizablefunctional groups as described hereinbefore. The spheres 80 aresynthesized to be monodisperse in size and are from about 10 nm to about10 microns. The spheres 80 are synthesized to possess thousands ofsurface functional groups that ionize in solution and cause the spheresto be highly charged as described hereinbefore. The interstices 82 areused to filter the submicron material through the openings between thespheres 80.

In sufficiently high concentration the spheres 80 form a face centered(FCC) or body centered cubic (BCC) lattice if the ionic strength of thesolution remains low. The lattice constant depends upon the spherenumber density and crystal structure formed. The choice of a BCC or FCClattice depends upon the form of the interaction potential betweenspheres. The crystalline colloidal array is then polymerized into aporous gel structure. The periodicity of the crystalline array isstabilized in a network that permits the diffusion of submicron materialthrough the interstices 82.

EXAMPLE 4

Crystalline colloidal arrays in the liquid phase are made out of 150 nmdiameter spheres. The colloidal array has (a) polymerizable monomerssuch as acrylamide, N,N'-methylene, bisacrylamide, N-vinylpyrrolidoneand (b) a polymerizing initiator such as benzoin methyl ether.Polymerization is initiated such as by UV light while the monomers aredissolved in the crystalline colloidal array medium. The elastic gelfilter membrane formed is a solvated, macroscopic, porous, cross-linked,polymer network. This elastic gel filter membrane contains submicronperiodic structures which can be tailored for use as smart materials.This gel filter membrane can be used as is or chemically altered. Thegel filter membrane can be further rigidized by adding additionalspecies to be polymerized.

In the passive membrane filter of this invention, the crystalline arrayis fixed in position by gelation and molecules smaller than the fixedinterstices can diffuse through the lattice of the gel membrane. The gelmembrane is extremely uniform in interparticle spacing and size cut-offand can be fabricated to any size desired by choosing an appropriatelattice constant and by selecting an appropriate sphere diameter. Thoseskilled in the art may readily determine the desired interparticleinteraction potential in order to maintain maximum colloidal ordering inthe presence of the disruptive forces associated with thepolymerization. See for example: Zahorchak et al., "Melting of colloidalcrystals: A Monte Carlo study", J. Chem. Phys., 96(9), May 1992;Rundquist et al., "Photothermal compression of colloidal crystals", J.Chem. Phys., 94(1), January 1991; Kesavamoorthy et al., "Colloidalcrystal photothermal dynamics", J. Chem. Phys., 94(7), April 1991;Rundquist et al., "Collective diffusion in colloidal crystals", J. Chem.Phys., 95(11), December 1991.

This embodiment also facilitates the use of a stack of these gelmembranes which serve as a size selective separation system as shown asa schematic cross section in FIG. 7. An array of membranes 84, 86 and 88are set up which allows retention by filtration of particles of any sizebetween about 5 Å and 100 microns with a very small deviation in thecut-off size. The sample inlet is designated 90 and the sample outletsare 92, 94 and 96. The material to be filtered is placed in the sampleinlet 90 and the filters 84, 86 and 88 are arranged so that the largersubmicron material is filtered first and continues by pressure flowuntil filter 88 filters the smallest desired submicron material.

In another embodiment of this invention, the gel membrane filters can bemade elastic to permit the filter to be stretched or compressed to alterthe interstices' size and shape. As shown in FIG. 8, stretching in thedirection of arrows A or compressing the membrane causes the latticeconstant to asymmetrically change. This stretching elongates theinterstices 82 along the stretched direction and permits the flow ofanisotropic molecules through the elongated interstices. The stretchingmay be effected by applying opposed forces to effect stretching in onedirection preferably by clamping opposite ends of the filter. Ifdesired, one may simultaneously stretch in a second direction as byclamping and applying force along an axis generally perpendicular to thefirst forces. The amount of applied force and whether stretching iseffected in one or two directions will be dependent on what shape orsize openings are desired. This embodiment is very useful for the highlysize selective separation of anisotropic large particles such as virusesor certain nonglobular proteins.

In another embodiment of this invention, a lattice generally similar tosodium chloride with positive and negative spheres is formed. Using thepolymerized lattice as a scaffold, spheres having an opposite chargefrom the lattice are diffused into the interstices as shown in FIG. 9.In FIG. 9, lattice spheres 100 are negatively charged. This lattice waspolymerized with a loose gel. Positive spheres 102 were then diffusedinto the negative sphere scaffold lattice 104. This shows the lattice ina closed position with the positive spheres restricting passage of thematerials through the openings.

In the embodiment described hereinbefore, there is a fixed lattice ofnegatively charged spheres and a mobile array of positively chargedspheres that can be controlled with an electric field as well known tothose skilled in the art. Alternatively, there can also be a fixedlattice of positively charged spheres and a mobile array of negativelycharged spheres controlled the same way. These mobile particles areselected from the group consisting of polymer and inorganic chargedparticles.

An equilibrium lattice generally similar to that of NaCl is formed.However, the relative size of the positive spheres can be made large orsmall compared to the negative spheres and the charge on the positivespheres can be independently varied.

The interaction potential between the dimer sites will be dipolar.Application of an electric field E in FIG. 10 (in a direction generallynormal to the paper) will cause the dipolar lattice sites to reorientbecause the negative spheres in this embodiment are fixed in position onthe lattice and the positive spheres are free to move. In oneapplication of this embodiment, an electric field E is arranged acrossthe membranes by applying a potential difference across electrodes onthe membrane surface. As shown in FIG. 10, application of the electricfield E causes the positive spheres to be removed from the intersticesof the lattice and to lie along an axis normal to the membrane lines ofnegative spheres and the material will be able to diffuse through theinterstices. Each negative shown is a line of spaced negativesprojecting into the paper. Each positive is a plurality of mobileparticles. Application of a potential generally in the place of themembrane will cause the positive spheres to reorient along the membraneand leave the interstices closed as was shown in FIG. 9. This is anexample of an electronically switchable (smart) membrane and can be usedas a size selective membrane filter.

In another application of this embodiment the electrodes are arrangedsuch that the positive spheres are caused to orbit the negative spheresand act as an active pump to pump fluid or small particles through theinterstices of the membrane. In order to cause the positive spheres toorbit the negative spheres, electrodes are set up which are always of anopposite charge to the positive spheres. These electrodes are switchedon to provide a force in a first direction due to opposite polaritycausing the positive spheres to move through a first portion of itsorbit and the electrodes are switched to another mode so as to cause theforce to move the positive particles through a second portion of thisorbit. These electrodes and switching means are well known to thoseskilled in the art.

In another embodiment the gel membrane containing spheres of about 10 nmto 10 microns is further hardened through backfilling with a polymerselected from the group consisting of acrylamide, N-vinylpyrrolidone,and N,N'-methylene bisacrylamide and this hardened material is thenemployed whole or broken into small pieces of at least about 10 micronsand mixed with a coating to give the coating refractive properties. Apaint made by this method would exhibit iridescence and a nail polishwould possess pearlescence.

It will be appreciated therefore that the invention has provided amethod for creating a solid filtering device which is capable of Braggdiffracting narrow bands of radiation. The disclosure includes severalembodiments and aspects of the method of the invention which providesfor versatility in preparing filtering devices for desired applications.

The invention also provides a method for creating gel membrane filtersto selectively filter submicron size material. These filters are basedon the self-assembly properties of crystalline colloidal arrays andincludes several embodiments and aspects of the invention which providesfor versatility in preparing gel membrane filtering devices to be usedin different applications. This is a method for making gel membraneswith fixed interstices. These filters may be used singly for filteringsubmicron material or arranged in a ladder array for filtering varioussize material on different levels. Another embodiment involves a methodfor making a gel membrane with flexible interstices so they can bestretched or compressed either mechanically or electromagnetically. Theinvention also includes the product, the gel membrane and the filtersthemselves.

Whereas particular aspects of the present invention and particularembodiments of the invention have been described hereinbefore forpurposes of illustration, it will be appreciated by those skilled in theart that numerous variations of the details may be made withoutdeparting from the invention as described in the appended claims.

What is claimed is:
 1. A method of filtering submicron size materialcomprising:establishing a self-supporting gel lattice membrane filterwhich is an ordered array of charged particles in a lattice;establishing said membrane filter with an interstice size less than orequal to the material to be filtered; and causing material containingsaid submicron size material to impinge upon said membrane filter,thereby resisting passage of said material through said membrane filter.2. The method of claim 1, including creating anisotropic interstices tofacilitate filtering anisotropic material.
 3. The method of claim 1,including employing interstices of submicron size.
 4. The method ofclaim 3, including using interstices having a maximum opening dimensionof about 1 nm to 10 microns.
 5. The method of claim 4, including usinginterstices having an opening with a maximum opening dimension of about1 nm to 1 micron.
 6. The method of claim 5, including employing aself-supporting gel membrane filter which is stretchable and alteringthe size of said interstices by stretching said membrane filter.
 7. Themethod of claim 6, including effecting said stretching by applyingmechanical forces to said gel membrane filter.
 8. The method of claim 7,including securing to said gel lattice membrane filter a plurality ofparticles having a first charge and introducing oppositely chargedmobile particles into said interstices of said gel lattice membranefilter.
 9. The method of claim 8, including employing an electromagneticfield to move said oppositely charged particles between an open positionwhich is substantially non-obstructing to said interstices and a closedposition which obstructs a substantially greater portion of saidinterstices.
 10. The method of claim 9, including employing positivelycharged mobile particles.
 11. The method of claim 9, including employingnegatively charged mobile particles.
 12. The method of claim 9,including employing mobile particles selected from group consisting ofpolymer and inorganic charged particles.
 13. The method of claim 9,including using for the particles of first charge, particles selectedfrom the group consisting of polystyrene, polymethyl methacrylate andsilica in the gel membrane filter.
 14. The method of claim 6, includingeffecting said stretching by applying an electromagnetic field to saidmembrane filter.
 15. The method of claim 14, including applying saidelectromagnetic field generally perpendicularly to the plane of saidmembrane filter.
 16. The method of claim 5, including employing the gelmembrane filter having fixed interstice size.
 17. The method of claim16, including employing a gel membrane filter wherein the intersticesretain material from about 5 Å to 100 microns.
 18. The method of claim5, including using spherical particles having an average diameter ofabout 10 nm to 10 microns.
 19. The method of claim 18, includingemploying a gel membrane filter having a thickness of from 10 nm to 1micron.
 20. The method of claim 1, including positioning a plurality ofsaid filters in relative close adjacency so that material sequentiallyflows through said interstices of said filters leaving on said filtersparticles of diameter of about 5 Å to 100 microns.
 21. A method offiltering submicron size material consisting of:establishing aself-supporting gel membrane filter which is an ordered array of chargedparticles in a lattice; securing to said lattice a plurality ofparticles having a first charge and introducing oppositely chargedmobile particles into the interstices of said gel lattice membranefilter; and employing said oppositely charged particles to pump liquidthrough said interstices of said lattice.